1. Field of the Invention
The present invention relates generally to subsea pipe connectors. More specifically, the present invention relates to a subsea riser stress joint.
2. Description of the Related Art
In offshore operations it is often necessary to provide piping that runs from the subsea equipment to the surface structures to provide communication between the subsea well and the surface structure. For example, a completed well may have a riser that extends from the subsea production equipment on the ocean floor to a wellhead on the surface structure. Such risers are typically supported in tension by the surface structure and affixed to the subsea equipment by a stress joint. The riser is subject to various stresses as it is suspended from the surface structure. Ocean currents, wave motion and other external forces may create large bending stresses in the riser. These stresses can cause failure of the stress joint between the riser and the subsea equipment.
Various systems have been proposed to prevent the failure of the stress joint between the riser and the subsea equipment. In one such system a complex mathematical formula describes the exterior profile of the stress joint so that stresses are carried over the entire length of the stress joint to prevent joint failure. Other systems describe simply tapered sections that serve the same function of distributing the stresses over a longer section of the riser. A major failure of these systems is that the stresses from the riser are transmitted to the subsea equipment, therefore requiring stronger subsea equipment to withstand the stresses. The subsea equipment must also be fairly large because the tapered sections are quite large at their lower ends. The prior art stress joints are also fairly lengthy. This results in additional costs.
A further problem not addressed by the prior art is that there is typically a weld where the stress joint is connected to the riser. The typical stress joint may have a section of riser welded to the upper end of the stress joint to facilitate connection to the riser string. The weld between the section of riser and the stress joint is more brittle than either the stress joint or the riser. The prior art stress joints do not adequately protect this weld from failure due to bending stresses.
It would be advantageous to have a stress joint that isolated the subsea equipment from some of the stresses incurred by the riser. It would also be advantageous to have a stress joint that isolated the weld used to connect the stress joint to the riser from some of the stresses incurred by the riser. It would be best if this stress joint was not so large as to require the subsea structure to be enlarged to accommodate it. Therefore, the art has sought a stress joint that is smaller in physical size and that can reduce the stresses transmitted to either the subsea structure or the connection weld.
In accordance with the present invention, a riser is suspended from a surface structure at an upper end such that a lower end is near subsea equipment. Between the subsea equipment and the riser a stress joint has a reduced diameter intermediate section to direct bending stresses away from the more brittle weld connection.
Another feature of the present invention is that the reduced diameter intermediate section directs bending stresses away from the subsea equipment. The stress joint of the present invention is also smaller and lighter than other similar stress joints.
The stress joint of the present invention is believed to increase the life of the weld joint between the stress joint and the riser as well as reduce fatigue on subsea equipment due to bending stresses. It may also be possible to use lighter duty subsea equipment due to the reduced size of the stress joint and the reduced bending stresses experienced by the subsea equipment.